The present invention relates to a driving circuit for a step motor which is suitable for carrying a rolled recording paper stepwise in a recorder such as a facsimile machine or printer.
In the case of a facsimile machine, for example, in order to record a picture on recording paper delivered from a feed roll, a step motor is used for a carrier mechanism to move the recording paper stepwise in the subscanning direction.
FIG. 1 represents a step motor driving circuit employed in the prior art for the carrier mechanism of the kind mentioned above. Each of the exciting coils 11-14 of a four phase winding wound on pole teeth of a stator (not illustrated) has one end grounded. The other ends of the four coils are connected to emitters of corresponding switching transistors 15-18 for on/off control of an exciting current for each coil. Collectors of the first and second transistors 15 and 16 are connected to a power line 21 through a resistor 19, and collectors of the third and fourth trnsistors 17 and 18 are also connected to the same power line 21 through a resistor 22. Control signals are inputted to the bases of the first to fourth transistors 15-18 from a control unit (not illustrated), thereby carrying out on/off control therefor.
When transistors 15-18 are rendered conductive in order, the coils 11-14 are excited successively, and a rotor (not illustrated) then rotates correspondingly to obtain a stable position in a changing direction of the excitation. The resistors 19 and 22 are used in the circuit to decrease an electrical time constant. Therefore response to an input signal is quick, control of rotational speed is comparatively easy, and open-loop control for which feedback is not required can be effected. On the other hand, an overshoot occurs at every step, and the carrier system of the recorder is subject to vibrations. When vibrations occur, a satisfactory recording will not be obtainable from the recording paper moving to a given position until the vibrations attenuate to some extent. Thus, if the system operates to insure a still time of the recording paper at each subscanning point for better recording, it will not be possible to obtain a high-speed recording. Another problem is that in high speed recording the vibrations accumulate from shifting to the next step operation before the complete attenuating of the vibrations of the step motor due to overshoot thus causing a step miss.
One proposal for solving the above problems includes a method to obtain a comparatively long still time for recording by moving the recording paper at the shortest possible time and reducing the vibrations of the step response. This method is represented by FIG. 2. A negative-phase clock (FIG. 2 at (b)) is generated between two positive-phase clocks (FIG. 2 at (a)), and an exciting step moving counter to the desired rotation is mixed into the motor when a state is changed. Vibrations of the step response are thus reduced as shown in FIG. 2 at (c). However, according to this method, a rotation vector is generated counter to the direction in which the rotor runs. Therefore the rotation torque of the rotor is not fully utilized.